A particular goal of Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) is to improve the Universal Mobile Telecommunications System (UMTS) standard. A 3GPP LTE radio interface offers high peak data rates, low delays, and an increase in spectral efficiencies. LTE supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD), which enables operators to take advantage of both paired and unpaired spectrum because LTE has flexibility in bandwidth (e.g., support for 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz bandwidths).
The LTE physical layer achieves higher data rates in part by using turbo coding/decoding and higher order modulations (e.g., 64QAM, 256QAM, etc.). The modulation and coding is adaptive based on channel conditions. Orthogonal Frequency Division Multiple Access (OFDMA) is used for the downlink, and Single Carrier Frequency Division Multiple Access (SC-FDMA) is used for the uplink. A particular advantage of such schemes is that the channel response is flat over a sub-carrier even though the multi-path environment could be frequency selective over the entire bandwidth. This reduces the complexity involved in equalization because a receiver may use simple single tap frequency domain equalizers. OFDMA enables LTE to achieve its goal of higher data rates, reduced latency, and improved capacity/coverage, all with reduced costs to the operator. The LTE physical layer supports Hybrid Automatic Repeat Request (HARQ), power weighting of physical resources, uplink power control, and Multiple-Input Multiple-Output (MIMO) antenna configurations.
Motivated by a growing number of LTE subscribers worldwide, the 3GPP developed an initiative that uses unlicensed spectrum with LTE alongside licensed spectrum, referred to as LTE-License Assisted Access (LTE-LAA). LTE-LAA enables operators to benefit from additional capacity available from the unlicensed spectrum, particularly in hotspots and corporate environments. With LTE-LAA, the extra spectrum resource, especially on the 5 GHz frequency band, can complement licensed band LTE operation.
FIG. 1 is an example schematic block diagram of a radio transmission circuit of a wireless network element. Circuit 10 includes baseband unit 14, RF chain 16, antenna ports 18, and scheduler decisions 20. Circuit 10 may, for example, comprise a radio transmission circuit of a wireless network element in an LTE or LTE-LAA wireless communication network.
The wireless network element may use a wireless communication protocol comprising several layers from the application layer down to the physical layer. Input bits 12 from upper layers are passed through baseband block 14, which typically comprises a channel encoder, interleaver and rate matcher, modulator, layer mapper, OFDM modulator, etc. (not illustrated). After the baseband signal is generated, it passes through RF chain 16 before it goes to antenna ports 18 for transmission. RF chain 16 typically comprises Digital to Analog Converters (DAC), In-phase/Quadrature (I/Q) imbalancers, oscillators, and Power Amplifiers (PA) (not illustrated).
Baseband signal generation depends on scheduler decisions 20 from upper layers, such as a layer 2 MAC. Scheduler decisions 20 are also influenced by feedback channel information 22 received over a feedback channel of a wireless receiver. For example, the wireless receiver can determine a suitable modulation and code rate at any given instance. As a particular example, when the receiver receives a good signal to noise ratio, the receiver might prefer a higher order modulation such as 256QAM or 64QAM, and when the receiver receives a low signal to noise ratio, the receiver might prefer a low order modulation such as QPSK or 16QAM.
In general, the power amplifiers in the RF block operate in the non-linear region to achieve good efficiency. An example amplitude-to-amplitude modulation (AM/AM) curve for a power amplifier is shown in FIG. 2.
FIG. 2 is a graphical illustration of an AM/AM curve for a power amplifier. Graph 20 charts a normalized input magnitude before power amplification on the x-axis and a normalized output magnitude after power amplification on the y-axis. Graph 20 shows that the input/output curve is highly non-linear. When the power amplifier operates in the non-linear region, some of the signals are leaked to the other frequency bands (i.e., adjacent carrier bandwidths).
FIG. 3 illustrates an example of spectral regrowth due to power amplifier non-linearity. Graph 30 includes power spectral density plots for non-linear power amplification 32 and ideal power amplification 34. FIG. 3 shows that the power spectral density plot is distorted, and there is a leakage of the desired signal to the adjacent channel bandwidths.
Adjacent Channel Leakage Ratio (ACLR) is used as a metric to measure the leakage due to non-linear power amplification. In FIG. 3, the ACLR with ideal power amplification 34 is around −100 dBc, while with realistic power amplification (with non-linearity 32) the ACLR is around −38 dBc.
One method to compensate for the non-linearity of the power amplifier is to distort the input signal to the power amplifier such that the output signal from the power amplifier is transformed to be close to what it would have been if the power amplifier would have been linear. An example of such a method is called a Digital Pre-Distortion (DPD) technique. In general, DPD may interchangeably be referred to as a signal linearization circuit, component, mechanism, or scheme.
FIG. 4 is an example schematic block diagram of a radio transmission circuit of a wireless network element with digital pre-distortion. Circuit 40 comprises baseband block 14 and RF chain 16 similar to those described in reference to FIG. 1. Additionally, circuit 40 comprises DPD block 42 and DPD extraction block 44.
In circuit 40, y1 is the output signal at the output of the power amplifier of RF chain 16, x1 is the output signal from the baseband block 14, and z1 is the input signal to the power amplifier of RF chain 16. This example considers the impact of nonlinear power amplification. In practical systems, the power amplifier of RF chain 16 may be preceded by many other blocks such as a DAC, local oscillator (LO), etc.
The output signal can be expressed as y1=f1(z1), where f1 is a nonlinear function which characterizes the power amplification. Using DPD, the above equations can be written as y2=f1(g1(x1)), where g1 is the function which characterizes DPD block 42. DPD extraction block 44 is chosen such that y2=f1(g1(x1))=G1·x1, where G1 is the gain of the power amplifier. The above equation shows that if g1 is properly chosen, the output of the power amplifier is linear.
FIG. 5 is a graphical illustration of an example spectral regrowth with DPD. Graph 50 illustrates the power spectral density plots for non-linear power amplification 52, ideal power amplification 54, and power amplification with DPD 56. FIG. 5 shows that the spectral regrowth is reduced when DPD techniques are applied. As illustrated, the plots for ideal power amplification 54 and power amplification with DPD 56 are nearly identical. ACLR in this example is around −100 dBc.